One example of a pressure monitor for a vehicle tire is disclosed in Japanese Laid-Open Patent Application No. 9-30220. This conventional pressure monitor comprises a signal sending device installed on a wheel of a vehicle for sending an electric wireless signal indicating a tire pressure at predetermined intervals, and a signal receiving device installed on a body of the vehicle for receiving the electric wireless signal sent by the signal sending device. Consequently, the conventional pressure monitor keeps watch on the tire pressure of the vehicle, and can issue an alarm when the tire pressure is less than a predetermined alarm tire pressure.
The signal sending device of the conventional pressure monitor is attached to the wheel, and a battery, which is a power source for the signal sending device, is contained in the signal sending device. Electric power is consumed when the signal sending device sends an electric signal. Therefore, it is desirable to restrain the signal sending frequency in order to lengthen the life of the battery. Consequently, the pressure monitor adopts such a control as, when the detected tire pressure changes a little bit, the signal sending device interrupts to send the electric signal of the detected tire pressure.
In such an above-mentioned control, however, in the case that the tire pressure, which is equal to or marginally higher than the alarm tire pressure, gradually decreases (at a small changing rate), the electric signal of the tire pressure will not be sent, even though the present tire pressure is lower than the alarm tire pressure. The signal receiving device, thus, does not issue an alarm in such a necessary situation.